Chemistry in the
Sunlight

by Jeannie Allen January 27, 2002

 

The sun’s awesome power drives a multitude of chemical reactions that are critical for life on Earth. In the atmosphere, ultraviolet radiation at wavelengths smaller than 242 nanometers splits molecular oxygen (two atoms bonded together) into atomic oxygen (individual atoms). Then when some energetically excited individual oxygen atoms encounter molecular oxygen, they can bond to form three-oxygen molecules, ozone.

Photograph of
Sunrise
Solar ultraviolet (UV) radiation drives the chemical reactions that produce ozone in both the Earth’s upper atmosphere (stratosphere) and its lower atmosphere (troposphere). (Photograph copyright National Center for Atmospheric Research Digital Media Catalog)

Ozone packs a punch in our lives that’s out of proportion to its small concentrations in the atmosphere. Its influence can be for good or ill depending on where it is. Ozone far above us in the upper atmosphere (stratosphere) absorbs and protects us from deadly ultraviolet radiation. Ozone in the lower atmosphere (troposphere) where we live is toxic. It reacts easily with biological tissue, donating oxygen atoms in the process known as oxidation. Breathing too much ozone over time impairs human lung capacity and causes illness and, for a few, premature death. Other animals and some plants also suffer from ozone overexposure. Several important crop plants such as soybeans and tobacco respond to currently common concentrations of ozone with lower rates of photosynthesis and reduced productivity.

Ozone formation occurs naturally throughout the atmosphere. From a few percent to as much as 50 percent of the ozone in the troposphere intrudes from the stratosphere. (The exact amount depends on location and time of year.) The rest forms from two groups of chemical compounds that occur both naturally and as by-products of fossil fuel combustion: nitrogen oxides (NOx) and volatile organic compounds (VOCs), which are carbon-containing gases and vapors such as gasoline fumes. Carbon monoxide also plays a critical role in some ozone formation reactions. Sunlight must be present for ozone to form, hence the term, “photochemical” smog. Without sunlight, no ozone forms.

As the human population has risen sharply and we have rapidly industrialized our economies over the last century, our consumption of fossil fuels has risen dramatically. Amounts of the byproducts of fossil fuel combustion emitted into the atmosphere have risen just as dramatically. Some of these byproducts contribute to ozone formation, therefore ozone concentrations in the air we breathe have risen as well. Ozone concentrations in the mid-1880s peaked somewhere around 10-15 parts per billion (ppb) in a given volume of air (Finlayson-Pitts and Pitts, 1999). Ozone levels in the troposphere now average 35-40 ppb around the globe in even the most remote regions (Finlayson-Pitts and Pitts, 1999; Fishman 1999; Madronich 1993). Typically in some suburban and rural areas in summer, ozone levels range from 80 to 150 ppb for several days at a time, far exceeding the U.S. National Ambient Air Quality Standard of 80 ppb averaged over an eight-hour period. According to the American Lung Association’s State of the Air 2002> report, unhealthy levels of ozone reached fully half of the American public during each of the last three years. In the most polluted urban areas of the world, ozone concentrations occasionally reach 500 ppb. (Finlayson-Pitts and Pitts, 1999

Map of Air
Quality

The air in many parts of the U.S. frequently contains unhealthy concentrations of ozone. This map from the Enivronmental Protection Agency shows ozone levels on August 12, 2002. Direct summer sunlight catalyzes emissions from cars and industry to create high ozone levels around many urbanized areas. (Map courtesy U.S. Environmental Protection Agency AIRNow)

Of all our common air pollutants, ozone has proven the most elusive to control. It forms through a highly complex series of reactions that take place over several hours, and that shift according to the presence of a multitude of other chemical species. Some governments have attempted to reduce ozone levels by mandating the reduction of hydrocarbons in motor vehicle emissions. But in recent years, it has become apparent that we must control NOx as well if we are to breathe healthy levels of ozone.

Photograph of Cloudtops
Understanding where ozone is likely to form and where it will travel means understanding the physical dynamics of the atmosphere: wind directions and speeds, humidity, and pressure. (Photograph courtesy NOAA Aircraft Operations Center)

Because tropospheric ozone forms over time, controlling it entails understanding the physical dynamics of chemical transport through the atmosphere: winds, humidity, atmospheric pressure, and so on. Since ozone and the chemicals that participate in its formation (precursors) can travel several hundred kilometers or farther on the wind, controlling ozone also requires monitoring and other research all around the globe. Since 1978, NASA and the European Space Agency (ESA), with the participation of Japanese and Canadian science organizations, have monitored tropospheric ozone using increasingly sophisticated satellite instruments. The Aura satellite will go into orbit in 2004 to make daily global maps based on high precision measurements of tropospheric ozone and its precursors.

next: Ozone, Space, and Time

  Ozone forms through a highly complex series of reactions that take
place over several hours.

 

Chemistry in the
Sunlight
 

 

Ozone, Space, and Time
Ozone concentrations in the troposphere vary widely over the Earth’s surface. The more direct the angle of sunlight, the greater its intensity. Where ozone’s precursors exist, more ozone tends to occur in regions closer to the Equator (lower latitudes) than in regions at the poles (higher latitudes).

Wind directions and speeds, high or low concentrations of NOx and VOCs, precipitation, and air temperatures influence ozone concentrations throughout the troposphere. Because ozone formation takes place over time, and winds can carry air parcels far downwind of NOx and VOC sources, people in some rural areas breathe more ozone than people in some urban areas.

Photograph
of Smog over Upstate New York
The STS-92 Space Shuttle astronauts photographed upstate New York at sunset on October 21, 2000. The view looks toward the southwest from southern Canada, and captures a regional smog layer extending across central New York, western Lake Erie and Ohio, and further west. Winds bring ozone and some chemicals that participate in its formation to rural areas downwind of emission sources. Ozone itself is invisible. (Photograph courtesy NASA JSC Gateway to Astronaut Photography of Earth)

Ozone and some of its precursors are intercontinental travelers. Some air pollution from North American reaches Europe, and pollution from Asia reaches western North America. Extensive biomass burning in South America raises ozone levels in Australia, and the same activity in Africa degrades air quality over the Pacific Ocean.

Ozone concentrations also vary through time, throughout the day and through the year. The highest ozone concentrations of the year generally occur during summer, when sunlight is most intense. On a daily cycle, as industrial and motor vehicle activity rises throughout the morning, concentrations of NOx and VOCs also rise. Ozone concentrations consequently reach maximum shortly after the peak in vehicle traffic, about noon or soon thereafter. Downwind from urban areas, ozone may peak later in the afternoon or even after dark. After sunset, when no more sunlight initiates ozone formation, ozone concentrations fall as ozone reacts with other chemicals and rapidly settles onto various surfaces. NOx and VOC concentrations drop as they too participate in other reactions.

Graph of NO2 and O3 over
Time

Measurements of nitrogen dioxide (NO2) [in blue] and ozone (O3) [in green] indicate rise and fall over a 48-hour period. Nitrogen dioxide participates in ozone formation, so after its concentrations peak, so do concentrations of ozone. Ozone concentrations peak during hours of maximum sunlight, around the middle of the day. (Graph ccourtesy William Brune, Penn State Earth Systems Science Center)

next: Chemistry of Ozone Formation
back: Introduction

  Ozone concentrations reach maximum shortly after the peak in
vehicle traffic, about noon or soon thereafter.

 

Chemistry in the
Sunlight
 

 

Chemistry of Ozone Formation
Ozone forms readily in the stratosphere as incoming ultraviolet radiation breaks molecular oxygen (two atoms) into atomic oxygen (a single atom). In that process, oxygen absorbs much of the ultraviolet radiation and prevents it from reaching the Earth’s surface where we live.

In the language of a simplified chemical formula,
O2 + sunlight yields O + O
When an electrically excited free oxygen atom encounters an oxygen molecule, they may bond to form ozone.
O + O2
yields O3
Destruction of ozone in the stratosphere takes place as quickly as formation of ozone, because the chemical is so reactive. Sunlight can readily split ozone into an oxygen molecule and an individual oxygen atom.
O3 + sunlight
yields O2 + O
When an electronically excited oxygen atom encounters an ozone molecule, they may combine to form two molecules of oxygen.
O + O3 yields O2 + O2
The ozone formation-destruction process in the stratosphere occurs rapidly and constantly, maintaining an ozone layer.

In the troposphere near the Earth’s surface, ozone forms through the splitting of molecules by sunlight as it does in the stratosphere. However in the troposphere, nitrogen dioxide, not molecular oxygen, provides the primary source of the oxygen atoms required for ozone formation. Sunlight splits nitrogen dioxide into nitric oxide and an oxygen atom.
NO2 + sunlight yields NO + O
A single oxygen atom then combines with an oxygen molecule to produce ozone.
O + O2 yields O3
Ozone then reacts readily with nitric oxide to yield nitrogen dioxide and oxygen.
NO + O3 yields NO2 + O2
The process described above results in no net gain in ozone. Concentrations occur in higher amounts in the troposphere than these reactions alone account for. In the 1950s, chemists discovered that two additional chemical constitutents of the troposphere contribute to ozone formation. These constituents are nitrogen oxides and volatile organic compounds, and they have both natural and industrial sources.

Nitrogen oxides (NOx) Nitric oxide and nitrogen dioxide are together known as NOx and often pronounced “nox.” Sources of NOx include lightning, chemical processes in soils, forest fires, and the intentional burning of vegetation to make way for new crops (biomass burning). NOx also come from smokestack and tailpipe emissions as by-products of the combustion of fossil fuels (coal, oil, and natural gas) at high temperatures. Coal-fired power plants are the primary sources of NOx in the United States. Automobiles, diesel trucks and buses, and non-road engines (farming and construction equipment, boats, and trains) also produce NOx.

Photograph of a refinery
Refineries generate large amounts of nitrogen oxides in the process of distilling gasoline and other petroleum products. Another major source is the burning of oil and gasoline in both power plants and automobiles. These nitrogen oxides form a link in a chain of chemical reactions that form ozone in the lower atmosphere. (Photograph copyright Philip Greenspun)

Volatile organic compounds (VOCs) such as hydrocarbons. “Volatile” refers to an extreme readiness to vaporize. Some plants and bacterial processes in soils emit VOCs. (The smell of a pine forest comes from a hydrocarbon called alpha-pinene.) VOCs also come from gasoline combustion and from the evaporation of liquid fuels, solvents and organic chemicals, such as those in some paints, cleaners, barbecue starter, and nail polish remover.

Photograph
of a House Painter
Evaporation of solvents and organic chemicals from some kinds of paint contribute volatile organic compounds (VOCs) to the air. VOCs participate in ozone formation. (Photograph copyright Philip Greenspun)

Ozone formation in the troposphere requires both NOx and VOCs. In a highly simplified version of tropospheric ozone-forming reactions,
NOx + VOC + sunlight
yields O3 (and other products)
The formula above represents several series of reactions that do not lend themselves to simple depiction. They involve the oxidation of VOCs in reactions that also involve NOx. Hydroxyl catalyzes some of the key reactions, and dozens of other chemical species take part. The result is ozone, nitrogen dioxide (available for more ozone formation), the regeneration of hydroxyl (available to catalyze more ozone formation), and some other chemical species. The specific ratio of NOx to VOC determines the efficiency of the ozone formation process.

Graph Showing Relationship
between Ozone Formation and NOx

The efficiency of ozone formation rises and then falls as the ratio of nitrogen oxides (NOx) to volatile organic compounds (VOCs) increases in an idealized plot. Higher NOx emissions result in less efficient ozone production. The modeler who made this plot intentionally left off all units of measurement, because the ratio itself is more important than the specific concentrations of the compounds. Authorities who want to control ozone production must take the ratio into account. (Graph courtesy Jim Meagher, NOAA Aeronomy Laboratory)

Photograph of
Cattle on the Range
Livestock such as cattle and hogs emit significant amounts of methane, one of the chemicals involved in ozone production. One cow produces an average of 0.23 kg (0.5 lb) of methane per day. Earth's total population of 1.4 billion cows produce 700 million kg (317 million pounds) of methane per day. Termite mounds and rice paddies are also significant producers of methane. (Photograph courtesy USDA On-line Photography Center)

Ozone formation with the hydrocarbon methane provides a useful example of the general pattern that most such reactions follow. The methane example is somewhat simpler and easier to follow than the others, described in steps.

Most ozone formations in the troposphere involve non-methane hydrocarbons. The chemistry of ozone formation from non-methane hydrocarbons follows the general pattern described above but is much more complex. NOx and VOCs together include about 120 different chemical compounds, and hundreds of chemical reactions can take place. Some of the participating chemicals may be intercepted part of the way through the process by reactions with other chemicals in the atmosphere, and may form intermediate compounds that act as temporary reservoirs for varying amounts of time.

Space
Shuttle Photograph of Cyclone over the Tasman Sea
Swirling cloud masses over the Tasman Sea between Australia and New Zealand illustrate the fluidity of our dynamic atmosphere. Winds and weather conditions such as air temperature and humidity influence ozone chemistry. (Photograph courtesy NASA JSC Gateway to Astronaut Photography of Earth)

An additional challenge arising for anyone tracking tropospheric ozone-forming reactions is that they entail interactions between different phases of matter (gas, liquid, and particles known as aerosols), and can occur on various kinds of aerosol surfaces in the atmosphere. Changing environmental conditions such as air temperature and humidity also affect ozone chemistry. Furthermore, many of the chemicals involved have very short lifetimes before they react with other chemicals to form new compounds. Scientists face myriad challenges in their pursuit of understanding tropospheric ozone chemistry.

References
Finlayson-Pitts, Barbara J. and Pitts, James N., Jr. 1999. Chemistry of the Upper and Lower Atmosphere. (Academic Press) P. 583

Fishman, Jack. 1990. Global Alert: The Ozone Pollution Crisis. (New York and London: Plenum Press)

Fishman, Jack, et al., NASA Langley Research Center, Hampton, VA. 1999. Surface Ozone Measurements: Exciting Science for All Seasons All the Time.

Madronich, Sacha. 1993. Tropospheric photochemistry and its response to UV changes. In The Role of the Stratosphere in Global Change. Vol. 18. NATO-ASI Series, Editor M-L. Chanin. (Amsterdam: Springer-Verlag) Pp. 437-61

Turco, Richard P. 1996. Earth Under Siege. (New York: Oxford University Press)

back: Ozone, Space, and Time
return to: Introduction

  In the 1950s, chemists discovered that nitrogen oxides and volatile
organic compounds contribute to ozone formation in the troposphere.

 

Chemistry in the Sunlight
 

 

Step 1. Sunlight splits ozone into an oxygen molecule and an oxygen atom.
O3 + sunlight yields O2 + O
An electronically excited oxygen atom reacts with water vapor to generate hydroxyl. (Oxygen and water molecules are abundant in the atmosphere, and so are available to take part in ozone-forming reactions.)
O + H2O yields OH + OH
Step 2. Hydroxyl reacts readily with other chemicals, and initiates another sequence of reactions. It combines with methane, producing water and a chemical called a methyl radical.
CH4 + OH yields CH3 + H2O
Step 3. The methyl radical combines with oxygen to produce methyl peroxy radical.
CH3 + O2 yields CH3O2
Step 4. The methyl peroxy radical combines with nitric oxide (from fossil fuel combustion) to produce a methyloxy radical and nitrogen dioxide.
CH3O2 + NO yields CH3O + NO2
Step 5. Sunlight splits the nitrogen dioxide into nitric oxide and atomic oxygen, which combines with molecular oxygen to yield ozone, as in Reactions 5 and 6 above.
NO2 plus sunlight yields NO + O
O + O2 yields O3
Some of the methyl oxy radical from Step 4 can participate in a different series of reactions that ultimately also make more nitrogen dioxide available for ozone formation. In this second series, the methyl oxy radical combines with oxygen to produce formaldehyde and a hyperoxy radical.
CH3O2 + O2 yields CH2O + H2O
The hyperoxy radical reacts with nitric oxide to yield hydroxyl and nitrogen dioxide.

Sunlight splits nitrogen dioxide into nitric oxide and atomic oxygen, which combines with molecular oxygen to yield ozone, as in Reactions 5 and 6 above (copied below).
NO2 plus sunlight yields NO + O
O + O2 yields O3
A third series of reactions, one involving formaldehyde produced in Reaction 12, can ultimately produce even more nitrogen dioxide that then becomes available for ozone formation. Sunlight splits formaldehyde into a formyl radical and atomic (single atom) hydrogen. Both of these species are extremely short lived and react almost instantaneously with molecular oxygen to form hyperoxy radicals.
CH2O + sunlight yields NCO + H
HCO + O2 yields CO + HO2
H + O2 yields H2O
The hyperoxy radical and nitric oxide then combine to form hydroxyl and nitrogen dioxide.
HO2 + NO yields OH + NO2
Reactions 5 and 6 then take place, resulting in more ozone.

Some Chemicals Involved in Ozone Formation:

Symbol Compound Name
CH2O formaldehyde
CH3 methyl radical
CH3O methyloxy radical
CH3 O2 methyl peroxy radical
CH4 methane
CO carbon monoxide
H hydrogen
HCO formyl radical
HO2 hyperoxy radical
H2O water
NO nitric oxide
NO2 nitrogen dioxide
NOx nitrogen oxides (NO & NO2)
N2O nitrous oxide
OH hydroxyl radical
O atomic oxygen
O2 molecular oxygen
O3 ozone

back: Chemistry of Ozone Formation